An LED light fixture is a physically distinct housing that contains one or more LEDs arranged in one or more “strings” (i.e., series connections). A room may be lit by multiple LED fixtures arranged at different points on (e.g.) the room's ceiling, and each fixture may have its own power supply to power the one or more strings therein. Each string in a fixture, even if designed to be identical to the other strings, may have different voltage and current requirements from the other strings in the fixture due to, e.g., manufacturing variations or dynamic noise. A power supply serving each string, therefore, must be account for the worst-case string, even if the other strings behave nominally. For example, if the LED strings are designed to require 20 Vdc, the power supply may be required to output 24 Vdc to account for variations in the current/voltage requirements of the strings. Every string that runs at an operating point other than the worst-case (24 Vdc) will, however, waste the additional voltage range, known as “headroom,” as heat. In many cases, every string runs at its nominal operating point (20 Vdc), and the entire headroom is wasted.
Some prior-art LED fixtures use a local power supply (i.e., one per fixture) and a technique called dynamic-headroom control to partially address this problem. Because the LED load is known and fixed (i.e., the fixture is manufactured with a certain number of LEDs/strings, and this number does not change) and because each string is directly connected to the local power supply, the power supply may adjust its voltage to reduce the amount of wasteful headroom. For example, the power supply may lower its output voltage until it senses that a string has reached its minimum operating voltage. These prior-art techniques are, however, dependent the power supply designer's a priori knowledge of the size and type of LED load and on direct control/monitoring of each string.
Having a single power supply serving multiple fixtures may be more economical than having a separate power supply for each LED fixture in a room. In this case, the power supply distributes a single power bus to a plurality of fixtures. The savings gained from sharing the power supply, however, may be lost to power wasted to unnecessary headroom applied to the fixtures. Prior-art dynamic-headroom techniques cannot be applied to the multi-fixture LED power supply at least because (i) the multi-fixture LED power supply cannot predict what kind or how many fixtures it will be required to power and (ii) the multi-fixture LED power supply cannot directly monitor or control each fixture (or each string in a fixture). Thus, a need exists for a way to dynamically adjust the headroom in a multi-fixture LED power supply.